Field of the Invention
The present invention relates to a composition for forming a p-type diffusion layer of a photovoltaic cell, a method for forming a p-type diffusion layer, a method for producing a photovoltaic cell, a paste composition, and a use of the composition. More specifically, the present invention relates to a technique for forming a p-type diffusion layer, which enables reduction in internal stress of silicon substrate serving as a semiconductor substrate, whereby damage to a crystal grain boundary can be suppressed and increase in crystal defects and warpage can be suppressed.
Description of the Related Art
A related art procedure of a silicon photovoltaic cell is described hereinbelow.
First, in order to realize high efficiency by promoting optical confinement effects, a p-type silicon substrate having a texture structure formed thereon is prepared, and subsequently subjected to a treatment at a temperature of from 800 to 900° C. for several tens of minutes under a mixed gas atmosphere of phosphorus oxychloride (POCl3), nitrogen and oxygen, thereby uniformly forming an n-type diffusion layer. According to this related art method, since diffusion of phosphorus is carried out using a mixed gas, the n-type diffusion layer is formed not only on the surface, but also on the side face and the rear surface. For these reasons, there has been a need for a side etching process to remove the n-type diffusion layer of the side face. Further, the n-type diffusion layer of the rear surface needs to be converted into a p+-type diffusion layer, and correspondingly an aluminum paste is applied to the n-type diffusion layer of the rear surface and then sintered to achieve conversion of the n-type diffusion layer into the p+-type diffusion layer and also formation of ohmic contact at the same time.
However, aluminum paste has low conductivity, and therefore, it is generally necessary to form a thick aluminum layer of about 10 to 20 μm after sintering on the entire rear surface in order to reduce the sheet resistance. Further, the coefficient of thermal expansion of aluminum is considerably different from the coefficient of thermal expansion of silicon, and therefore, such a difference results in generation of large internal stress in the silicon substrate during the sintering and cooling processes, which contributes to damage to a crystal grain boundary, increase in the crystal defects, and the warpage.
In order to solve this problem, there has been a method to reduce the thickness of the rear surface electrode by decreasing the amount of a paste composition to be coated. However, when the coating amount of the paste composition is decreased, the amount of aluminum diffused from a surface of a p-type silicon substrate into an internal portion is insufficient. As a result, a desirable BSF (Back Surface Field) effect (an effect in which collection efficiency of generated carriers is increased due to the presence of a p+-type layer) is not achieved, resulting in the problem of a decrease in properties of a photovoltaic cell.
For these reasons, for example, there has been proposed a paste composition including an aluminum powder, an organic vehicle, and an inorganic compound powder whose coefficient of the thermal expansion is lower than that of aluminum, and whose at least one of melting temperature, softening temperature and decomposition temperature is lower than the melting temperature of aluminum (for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-223813)